Phenotypic variation in Galerida larks in Morocco: the role of history and natural selection

The relative influences of history, natural selection and hybridization in shaping phenotypic variation in closely related taxa is a crucial issue in current evolutionary biology. In this study, we used as a model two sibling but paradoxically highly variable species of larks (Galerida theklae and Galerida cristata) of Morocco to separate the impacts of these evolutionary forces. In the former species, variation is manifested mainly in colouration, while in the latter, variation also encompasses bill size and shape. Mitochondrial and nuclear DNA sequencing were used to identify the historical relationships among the subspecies and species. According to our analyses, G. cristata and G. theklae diverged about 3.7 million years ago (Ma), and we found no evidence for a role of hybridization in maintaining their similarity. In G. theklae, there was no further subdivision, while in G. cristata two major mtDNA groups were identified (divergence approximately 1.1 Ma). These two lineages are parapatric and regroup, respectively, the three short-billed subspecies [G. (cristata) cristata] and the two long-billed subspecies [G. (cristata) randonii]. Patterns of morphological variation were then contrasted to this pattern of neutral relationships: we found that G. (c.) cristata was morphologically more similar to G. theklae than to G. (c.) randonii. Overall, these results point towards the prominent role of (i) natural selection and/or phenotypic plasticity in adapting the plumage to local conditions and (ii) natural selection in combination with historical isolation in driving the divergence in size and bill morphology in the crested larks.